help with sticking cusor in mail?

when i write mail and i get ready to send it i go to the first misspelled word and correct it. then i use the right arrow to move to the next word and check structure along the way. but the cursor sticks and jumps, sometimes complete lines and sentences. anyone have a fix. i use firefox as my browser but i use it on 4 computers and this is the only one that does this. it did not do in this mail. thanks